In the series finale, the Department of Defense's closure of Eureka is delayed by random wormhole materializations.
The Astraeus participants make their farewells to their earthbound family and friends. Elsewhere, Carter and Jo investigate the person—or thing—which attacked the presidential convoy and if it is in any way related to the cosmic anomalies threatening Eureka.
Tess' reign as head of Global Dynamics may be shorter than planned after she receives an offer from Australia. Elsewhere, the town deals with a magnetic disturbance and Carter must come to terms with Zoe leaving for college.
Carter starts to swap bodies uncontrollably, which puts both his body and his consciousness in awkward situations. Meanwhile, Warren Hughes returns to GD. This time to evaluate Astraeus crew members' reactions to their abductions and any effect to their job performances. But Carter's body swapping jeopardizes jobs, relationships and, ultimately, his own life. Elsewhere, Zane's quest for answers from the matrix reveals disturbing results for both him and Fargo.moreless
Carter and Jo continue their search for the missing Astraeus crew. A surprising source confirms the new theory of the disaster, and offers to help them rescue the crew. Meanwhile, back at the lab, lives are at risk as more of the crew comes closer to the truth of their situation, and Carter is sent in on what may be a one way virtual rescue mission.moreless
Henry's 20-year-old project returns with more on board than it left with, and when a man is killed and another injured, Henry will have to face his past in order to save everyone's life. Back in town, Jo and Carter attempt to return the items from the towers to the owners with mixed results.moreless
Carter investigates the deaths of scientists who drowned in less than liquid circumstances. Meanwhile, Allison's baby shower goes green.
Global Dynamics goes into lockdown to protect itself from the now airborne metal-eating bacteria. Stuck inside GD, Sheriff Carter and Stark must work together to save both the town and Kevin, Allison's son.
While under the influence of a smart drug, Carter becomes a genius like everyone else in town and starts to change the laws of physics. Unfortunately, his condition soon endangers all of Eureka, including Allison's brother who is in town just to meet his new brother-in-law.
After Senator Wen calls off the search, Kevin designs a tool to find the Astraeus ship that is still missing after four months. Unfortunately, the device's assembly stops after Andy and Carter are arrested while stealing a component for it. Elsewhere, Jo's walkabout is rocky, and at the Astraeus crew's location Dr. Marten posits a dangerous theory about their situation.moreless
The doubles steadily take over GD, which means Fargo, Allison and Zane are in a fight for their lives and everyone else's as they try to avoid capture and fix the problem.
Carter deals with a bank robbery which may relate to the sudden gravity issues besetting the citizens of Eureka. Meanwhile, Fargo's rivalry with Isaac Parrish is boosted when he learns Holly and Parrish went on a date.
When the ship with the Astraeus crew returns to earth, its occupants discover that four years have elapsed and time isn't the only thing that has changed.
When the bowling team from Area 51 hits town, the long-standing rivalry-cum-war is reignited, and a prank goes horribly wrong. Now Carter not only has to mediate between Area 51 and the home team to prevent more destruction, but also deal with the consequences of the first prank: a blob with an insatiable appetite. Elsewhere, Duncan returns to plead his case to a resistant Lexi.moreless
The Department of Justice arrives to investigate Grace, and Henry struggles with his conscience. Holly receives a message that triggers a chain of events. The question is whether it's her actions or Fargo's malfunctioning signature achievement that are causing the scary incidents all over GD and the town.
Attacks in town may scuttle the Astraeus mission in the same week the final 20 will be named. Carter and Allison appeal the auditor's recommendation, but the proposed fix, a memory-reading gadget for the candidates' final interviews, may give them more details about each other than desired.
When an explosion in the Astraeus lab accidentally sends Andy to Titan, the team must devise a plan to save him before it's too late.
As Carter and Blake prepare a Christmas surprise for the kids, a wave of energy sweeps over Eureka, transforming everyone into animated characters.
Carter can't get into the Christmas spirit especially with the town surrounded by a force-field. Fargo organizes the GD holiday party, and Allison is full of holiday cheer.
Carter and Allison's perfect honeymoon is interrupted when sabotage is discovered in Eureka. Elsewhere, Holly faces another hurdle to remain intact.
It's disaster drill day at GD and everyone must participate in order to see how well they would do in a disaster. Unfortunately the computer conducting the drill makes the simulation a reality. Elsewhere, Fargo undergoes risky interfaces with the matrix to confirm Zane's findings.
The Astraeus crew members attempt to overcome their experiences in the matrix; Fargo deals with Holly's death, Grace struggles to maintain her relationship with Henry, and Allison fights her feelings about Jo as they share Carter's house. This all fades to background noise, however, when Dr. Parrish's latest project is accidentally released and threatens to ignite the whole town.moreless
The Department of Defense insists on installing new security and removing all Astraeus artifacts, but Fargo resists because he believes Holly is alive in the matrix. Elsewhere, Henry recruits Carter for a brain experiment.
Carter and Allison's romantic plans run afoul of a rogue submarine, and it's up to Jo to save their lives … if she can fix the mess at Global Dynamics caused by an out-of-control prankster in time.
Tiny makes an explosive reentry on the same day the Department of Defense demonstrates how little control Allison and Carter have over their personal lives by sending an auditor whose task it is to analyze their relationship. Elsewhere, Jo turns herself into a wedding planner extraordinaire for a reluctant Grace and Henry as Zane prolongs his departure, and Fargo must decide how he wants his relationship with Holly to progress. But none of it matters when it appears the gases Tiny brought back may be be the end of Eureka.moreless